1. Field of the Invention
The present invention relates to a mark detection method and a mark position detector which are suitable for detecting a mark position of an alignment mark formed on a substrate such as a wafer, to which a mask pattern is transferred, in a lithographic process for making a semiconductor device, a liquid crystal display element, a thin-film magnetic head or the like.
2. Description of the Related Art
In making semiconductor devices etc. in the past, the reductive projection and batch exposure type of projection exposure apparatus (stepper) was often used to transfer a pattern on a reticle as a mask to each shot area on a wafer (or a glass plate or the like) as a substrate which is coated with photoresist. Recently, the scanning exposure type of projection exposure apparatus, too, has come into use, which may be the step-and-scan system for exposure with a reticle and a wafer moved synchronously at a speed ratio which is substantially the magnifying power of projection.
In general, a semiconductor device is formed with minute patterns stacked in several tens of layers in predetermined positional relationship with each other on a wafer. Therefore, in order to align a reticle pattern for exposure precisely with the circuit pattern or patterns formed already on the wafer, the projection exposure apparatus is provided with an alignment sensor for detecting the position of an alignment mark (wafer mark) formed together with the circuit pattern or patterns on the wafer.
Various systems are known as alignment sensors. As an alignment sensor for highly accurate position detection, the grating alignment system is used, which radiates laser light over almost the whole area of an alignment mark in the form of a grating and receives the diffracted light from it. The alignment mark is formed as a brightness grating or a recess/protrusion grating in the previous processes of exposure, film formation and etching. In the past, the wavelength of the laser light used for the grating alignment system was set in a wavelength band of approximately 550-780 nm, by which the photosensitive material such as photoresist on a wafer was not be exposed.
By the way, in order to form minute circuit patterns more precisely, as semiconductor integrated circuits have been more miniaturized or minute, flattening processes have been adopted which involve flattening the outer surfaces of the layers formed on wafers. A typical example of the flattening processes is the CMP (chemical and mechanical polishing) process for polishing the outer surfaces of formed films to make them almost completely flat. The CMP process has often been applied to the interlayer insulation films (dielectrics such as silicon dioxide) between the wiring layers (metal) of semiconductor integrated circuits. In such a case, because a flattened interlayer insulation film transmits a beam of light with a wavelength of approximately 550-780 nm, and the beam reaches the alignment marks on the layer under the film, it has been possible to use alignment sensors on the conventional grating alignment system.
In this connection, the STI (shallow trench isolation) process has recently been developed, which includes a step of forming shallow trenches or grooves of predetermined width in a layer to isolate or insulate, for example, a minute element from an adjacent minute element thereto, and the step of embedding dielectrics or other insulators in the trenches. The STI process may also include a step of flattening by the CMP process the outer surface of the layer in which the insulators have been embedded, and forming a polysilicon film on the flattened surface. In this case, no recesses nor protrusions corresponding to the alignment marks of the layer under the polysilicon layer are formed in the outer surface of the polysilicon layer. A polysilicon layer does not transmit any beams of light with a wavelength of 550-780 nm (visible light). Consequently, the sensor with the conventional grating alignment system was not able to detect an alignment mark formed in the layer under the polysilicon layer. Therefore, there has been need for an alignment sensor which can precisely detect through a polysilicon layer etc., for example, on the grating alignment system, the position of a flat alignment mark formed particularly by the STI process.
In view of the foregoing points, an object of the present invention is to provide a mark detection method and a position detector which allow precise detection of the position of a flattened alignment mark even if the mark is formed at the bottom of a film which transmits no visible light.
Another object of the invention is to provide an exposure apparatus provided with such a mark position detector.
A further object of the invention is to provide an exposure method and a method of making a semiconductor device which allow accurate alignment by using the mark detection method.